The Sydney Roosters' premiership defence received a hammer blow on Saturday with confirmation Sonny Bill Williams will miss up to a month of NRL action with a fractured thumb.

Already reeling from an embarrassing 16-12 loss to Newcastle on Friday night, the fallout continued with the superstar back-rower's injury.

Williams is set to be back ahead of the NRL finals, with his scheduled return in round 25 against Melbourne.

Roosters chief medical officer Ameer Ibrahim played down fears Williams might end his NRL stint on the sidelines.

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"He let us know (about the injury) in the second half but he was happy to continue on with it until the end of the game," Dr Ibrahim told Fairfax Media.

"It's an injury that doesn't need surgery. I imagine plaster and splinting and, when he feels good (he'll return).

"There is part of the thumb which is a more serious injury than others," Ibrahim said. "It wasn't that area. It wasn't a season-ending injury and we just did scans this morning that confirmed that it wasn't an injury which needed surgery."

In a further blow to the Roosters, utility Mitchell Aubusson will be sidelined for up to two weeks with a high ankle sprain which forced him off the ground against the Knights.

The loss was the third from the Roosters' past four matches and left them on 24 points in fifth spot on the ladder.

Coach Trent Robinson's men face St George Illawarra at Allianz Stadium on Saturday with the likes of hulking forward Kane Evans in the frame for a call-up after some outstanding performances for feeder club Newtown.

Before Friday night's loss in Newcastle, the Roosters had already been struggling without NSW State of Origin stars, centre Michael Jennings and winger Daniel Tupou.

With six regular-season games remaining, the tricolours face a big test of their depth and character.

After next weekend's clash with Dragons, they host Gold Coast, play the Wests Tigers away, travel to New Zealand to take on the Warriors, then finish with home matches against the Storm and South Sydney.

Poll: Does Paul McGregor deserve to become St George Illawarra's permanent coach?

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Please select an answer. Yes - he's done a good job and will make the team better